The present patent application is related to fiberoptic networks, and, in particular, optical transmitters for WDM and DWDM network systems.
In many modern fiberoptic networks, the wavelength, and hence the frequency, of the optical signal plays an important role. In WDM (Wavelength Division Multiplexing) fiberoptic networks, for example, optical signals are sent at predetermined wavelengths over optical fibers. Each predetermined wavelength, as defined by the ITU (International Telecommunications Union), forms a communication channel in the network. An advanced version of WDM networks is the DWDM (Dense Wavelength Division Multiplexing) network in which the number of wavelength channels is increased by reducing the channel wavelength separation. It should be noted that the term, DWDM, is used hereafter to refer to both WDM and DWDM networks, and other fiberoptic networks which rely upon wavelength to define communication channels, unless indicated otherwise.
Another important parameter of the optical signals in an fiberoptic network is power. With many different wavelength channels being carried on a single optical fiber, it is desirable that the power of the different channel signals be controlled to compensate for the spectral nonuniformity of the fiber and other non-uniform losses in the network. Conventionally, channel power control has been achieved by an external variable optical attenuator (VOA) in the optical path for each channel, but the addition of the VOA to the system architecture has undesirable consequences, such as added optical insertion loss, reduced reliability, added cost and complexity, and the consumption of valuable space in the optical transmitter system in the network. Hence it is desirable to control power at the transmitter itself. This has been done typically by changing the forward bias current of the semiconductor lasers which are commonly used in network optical transmitters. This technique alone, however, is not practical since the change in the bias current also causes a change in the output wavelength of a semiconductor laser. Thus action moves the transmitter out of the ITU tuning grid, the selected DWDM channel wavelengths, and out of the wavelength locking range. An iterative process must be used to obtain the desired power at the desired wavelength.
On the other hand, to better control output wavelength and power, the present invention utilizes an algorithm based on a closed mathematical form for the physical relationships between a laser""s optical power, wavelength, bias current and temperature for a multifunction optical WDM transmitter. Control over the power of wavelength channel signals without changing the output wavelength is efficiently effected, among different applications of the present invention.
The present invention provides for a method of operating a laser transmitter system for a DWDM network. A semiconductor laser generates an output signal for the laser transmitter system. The output power and wavelength of the semiconductor laser are targeted by setting a bias current and operating temperature of the semiconductor laser according to a closed mathematical form relating variables of output power, wavelength, bias current and operating temperature of the semiconductor laser. The variables are related to each other by empirically determined coefficients. The closed mathematical form relating the original operating condition to a different condition follows the equation:
Itho[exp(xcex94T/To*xe2x88x921)]+1/xcex7o[P1 exp((xcex94T/T1*)xe2x88x92Po]+(xcex94xcexd+xcex1txcex94T)/(xcex1i+xcex2ixcex94T)=0
where xcex94T is the change in temperature, P1xe2x88x92Po is the change in output power, and xcex94xcexd is the change in frequency, and Itho, To*, xcex7o, T1*, Po,xcex1t, xcex1i, and xcex2i are the empirically determined parametric coefficients. The subscript o always represents the original conditions, whereas 1 refers to desired new conditions. To is the original or preset temperature, typically set at absolute temperature of 293xc2x0 K to 298xc2x0 K; Itho is the original or preset threshold current at the preset temperature To; xcex7o is the power slope efficiency at To; xcex94xcexd is the desired difference in frequency; Po is the power at To and Io; xcex1t is the frequency change, with respect to temperature variation at xcex94I=0; xcex94I is equal to I1xe2x88x92Io; xcex94T is equal to T1xe2x88x92To; xcex1i is the frequency change with respect to current variation at xcex94T=0; and xcex2i is the rate of change of xcex1i with respect to temperature change.
Another equation of the closed mathematical form is:
xcex94I=xe2x88x92(xcex94xcexd+xcex1txcex94T)/(xcex1i+xcex2ixcex94T)
According to the present invention, the output wavelength of the semiconductor laser can be kept constant as its output power is varied. Likewise, the output power can be kept constant as the wavelength is varied, or both the output power and the wavelength can be varied.